PROFESSOR OLMSTED'S OBJECTIONS. 435 



there can be no particular patches, on the surface of the 

 earth, and especially on the surface of the sea, so much 

 heated above surrounding regions, as to give rise to these 

 upmoving columns of such tremendous power." 



These objections have some plausibility, but it is a plau- 

 sibility arising entirely from a misconception of the doctrine 

 which I teach. In the first place, it does not follow, that 

 because tornadoes sometimes occur in the night, the clouds 

 themselves which formed these tornadoes, commenced form- 

 ing in the night. These clouds might have formed many 

 hours before they assumed the tornado character, and at a 

 great distance from where they were first observed. And 

 indeed if my doctrine is true, it must have been so ; for it 

 requires considerable time to form a cloud of such great 

 perpendicular height as to make a tornado cloud, and dur- 

 ing all this time, the cloud has a motion of translation. 



It is manifest that when cloud is once formed, and comes 

 over a region where the air at the surface of the earth is 

 colder than that just above it, as it will be in the night, still 

 from the great levity of the cloud, when compared with the 

 air around the cloud, the whole column of air, even down 

 to the surface of the earth will be forced upwards, by the 

 superior weight of the surrounding air ; and if there is but 

 little difference between the temperature of the air and the 

 dew point, when the cloud comes over a place, as it passes 

 along over the surface of the earth even in the night, the 

 air under it, being relieved from some of the pressure which 

 was on it before the arrival of the cloud, will expand, and 

 by the cold of expansion, condense a portion of its vapor into 

 cloud, and thus the cloud will reach the earth. 



Moreover there is a tendency in one tornado to form an- 

 other, and the second has a tendency to form a third, and 

 so on till there is a forest of them all in operation at the 

 same time ; and the annulus of each tornado will keep them 

 apart for a short time. For as the barometer stands above 



